1. Field of the Invention
The present invention concerns a method of forming a pattern by using a phase shifting mask. The present invention can be utilized as a technique for forming various kinds of patterns and, for example, it can be utilized when forming various kinds of patterns such as a resist pattern in a semiconductor device production process.
2. Description of the Prior Art
The fabrication size of devices to which a pattern is formed by using a photomask, for example, semiconductor devices, has been made finer year by year. With such a situation, in a photolithographic technique for obtaining miniaturized semiconductor devices, a so-called phase shifting technique which provides a phase difference to light permeating a mask, thereby improving the profile of a light intensity, has been highlighted in order to further improve the resolution power of the photolithography.
The phase shifting technology has been described, for example, in Japanese Patent Laid Open Sho 58-173744, "Improving Resolution in Photolithography with a Phase-shifting mask" by Marc D. Levenson, et al., IEEE Transactions on Electron Devices, Vol. ED-29, No. 12, December 1982, P 1828-1836, and "The Phase-shifting Mask II: Imaging Simulations and Submicrometer Resisto Exposures" by Marc D. Levenson, et al., IEEE Transactions on Electron Devices Vol. ED-31, No. 6, June 1984, O 753-753.
Further, Japanese Patent Publication Sho 62-50811 discloses a phase shifting mask having a predetermined pattern comprising permeable portions and impermeable portions, in which a phase member is positioned at least to one of the permeable portions on both sides of an impermeable portion, and a phase difference is caused between the permeable portions on both sides thereof.
A description will now be given of the phase shifting technology known so far with reference to FIGS. 18a, 18b below.
For example, in a case of forming a line-and-space pattern, in a usual existing mask a light shielding portion 10 is formed using a light shielding material such as Cr (chromium) or other metal or metal oxide on a transparent substrate 1 made of a quartz substrate or the like as shown in FIG. 18(a) by which a repeating line-and-space pattern is Formed as an exposure mask. Theoretically, in the intensity distribution of light transmitting the exposure mask, the light intensity is 0 at the light shielding portion 10, while light transmits at other portions (permeable portions 12a, 12b) as shown by reference A1 in FIG. 18(a). Considering then one permeable portion 12a, the transmission light incident on an exposed material has a light intensity distribution by diffraction of light or the like having hill-like extremes at the skirts on both sides as shown at A2 in FIG. 18(a). The transmission light A2' from the permeable portion 12b is indicated by a dotted line. When light from each of the permeable portions 12a, 12b is synthesized, the light intensity distribution loses its sharpness and the image becomes blurred due to light diffraction as shown at A3. As a result, sharp exposure can no more be attained. On the contrary, if a phase shifting portion 11a (made of material such as SiO.sub.2 or resist, referred to as a shifter) is disposed on every other light permeable portion 12a, 12b of the repeating pattern as shown in FIG. 18(b), a blur of the image caused by the light diffraction is offset by the inversion of the phase. Accordingly a sharp image is transferred to improve the resolution power and the focus margin. That is, as shown in FIG. 18(b), in a case where a phase shifting portion 11a is formed at the permeable portion 12a on one side as shown in FIG. 18(b), and, if it provides a 180.degree. phase shift, for example, the light passing through the phase shifting portion 11a is inverted as shown by reference B1. The light from the adjacent permeable portion 12b causes no such inversion since it does not pass the phase shifting portion 11a. Portions of light distributions are inverted to each other in phase and are offset with respect to each other at a position shown by B2 in the figure at the skirts of the light intensity distribution, and, the distribution of light on the exposed material have sharp identical shape as shown at B3 in FIG. 18(b).
In the above-mentioned case, for making the effect most reliable, it is most advantageous to invert the phase by 180.degree. and, for this purpose, a phase shifting portion 11a has a film having a film thickness D: ##EQU1## in which n represents a refractive index of the phase shifting portion and k represents a wavelength for exposure light.
In the pattern formation by exposure, means used therefor are sometimes referred to, for example, as a reticle when it is used for diminishing projection, as a mask for 1:1 projection, as a reticle when it corresponds to an original, or as a mask when it is reproduced from the original. In the present invention, masks or reticles in such various meanings are collectively referred to as a mask.
The phase shifting mask described above for shifting the phase of light between adjacent light permeable portions (ideally 180.degree. inversion) is referred to as a space frequency modulation type (or Levenson type), and which has the greatest effect for improving the resolution power among other several kinds of phase shifting mask methods (referred to as an edge emphasis type, light screening effect emphasis type or the like)(refer to "Nikkei Micro-device", July 1990, P 108-114, Nikkei MacGrow Hill Co.).
The phase shifting mask of this type is a highly prospective technique for improving the resolution power, and it is necessary to provide, desirably, a 180.degree. phase difference between transmission lights in adjacent patterns as described above. However, portions with the phase difference of 0.degree. to 180.degree. can not be satisfactorily arranged adjacent to each other depending on the pattern and, accordingly, applicable patterns have been restricted.
For instance in a case where there are first and second patterns P1 and P2 adjacent to each other, and further there is present a third pattern P3 adjacent with both of the patterns P1 and P2, 0.degree. and 180.degree. can not be basically allocated. If 0.degree. and 180.degree. are allocated to the first and the second patterns P1 and P2 adjacent to each other, the phase shifting effect can not be provided to either one of the first and the second patterns P1 and P2 in or none of the case of allocating 0.degree. or 180.COPYRGT.to the third pattern P3 adjacent to both of the patterns P1 and P2.
Further, there is also a technique of attaching a sub-pattern at the periphery of a contact hole or an isolated line, and providing the sub-pattern with a phase difference of 180.degree. to improve the resolution power. For instance, as shown in FIG. 11, four sub-patterns 11 are formed around a pattern 12 for forming a contact hole, and the sub-patterns are used as the 180.degree. inversion phase shifting portion. However, such a technique also has a drawback that sub-patterns as the phase shifting portion can not be arranged if the patterns are excessively close to each other.